Non Patent Literature 1 discloses a method for enabling judgment of whether an object appearing in an image is a person or not. In that judging method, a process of judging whether an object is a face or not is repeatedly performed on an entire image to detect a face in the image. It can judge whether or not an object in an image is a person by setting a human body to instead of a face as the detection target.
Non Patent Literature 2 discloses another method for enabling judgment of whether or not an object appearing in an image is a person. In that judging method, a process for evaluating the difference between an image that was taken at a certain time (background image) and a current image, using a small amount of processing, is used. The method rapidly detects a partial area that is likely to contain a moving object based on the difference, and, only in the case of an image in the partial area, judges whether the image is a ‘person’, an ‘automobile,’ or ‘another object’. That is, the method reduces the number of times for performing the process that judges whether the image contains a ‘person’ or a ‘non-person’ by rapidly narrowing down an area to a partial area that is likely to contain the object.
Although the judging method described in Non Patent Literature 2 uses a relatively simple judging process by using the feature quantity which is calculated from shapes or the like obtained from the image, it can provide higher judging performance with a slightly longer processing time by being combined with a statistical image recognition technique described in Non Patent Literature 3, for example.
The technique described in Non Patent Literature 1 features rapidity, which is only distinguished when it is compared with a conventional technique for repeating a pattern recognition process. When the technique described in Non Patent Literature 1 is compared with a technique for only performing a calculation to determine the difference between images as described in Non Patent Literature 2, the latter technique for performing a calculation to determine the difference seems to require a small amount of processing.
It is easily thought that whether an object is a ‘person’ or a ‘non-person’ can be easily determined by judging whether the object ‘has the head or not’ on that assumption that a human body has a head portion. For example, since the face is a portion of the head, the face searching process described in Non Patent Literature 3 is virtually capable of judging whether an object is a ‘person’ or a ‘non-person’.
In general, if the head occupies a comparatively large portion of an image, the face judging process for judging whether it is a ‘face’ or a ‘non-face’ has higher judging efficiency than the process of judging whether it is a ‘person’ or a ‘non-person’. In addition, the face judging process is capable of providing an accurate judgment even if portions of the human body other than the face are hidden. It is easily expected that the face judging process can be speeded up to some extent if it is used in combination with a technique for rapidity detecting a partial area that is likely to contain an object as described in Non Patent Literature 1 as preprocessing.
Patent Literature 1 describes another process of judging whether an object is a ‘face’ or a ‘non-face’. According to the method described in Patent Literature 1, in the case of an object that is captured by Laser Rader, a determination is made whether this object is a ‘person’ or a ‘non-person’ by judging whether the image of the area that contains the object ‘contains a face or not’.
Patent Literature 2 discloses a technique that uses stereo vision taken by cameras to measure distances from the cameras to an object, and based on the measurements, narrows down to the size of the potential face on the image, instead of merely narrowing down the area to search a face. According to the technique, the amount of processing that is needed to search for a face can be reduced.
Patent Literature 3 describes a method for extracting a face area corresponding to the face of a person in an image, estimating the height of the face area from the floor of the center of gravity with reference to camera parameter, and from the estimated result, extracting a wheel chair area in the image.